Methods in principle for conversion of light to electrical current (and vice versa) by means of organic electronics have been known for several decades. An industrial breakthrough has been accomplished by multilayer constructions which are currently on the point of readiness for the mass market, as shown schematically, for example, in figure I for an organic light-emitting diode (OLED) or in figure II for an organic solar cell. Even though the efficiency of these components in the last few years in particular has undergone a distinct rise in performance through the use of optimized classes of organic compounds, promising approaches are still resulting in even higher quantum yields and hence even higher efficiencies with lower material costs.
One of these approaches lies in the use of phosphorescent emitters, called triplet emitters, which find use, for example, in PHOLEDs (phosphorescent organic light-emitting devices). In view of the applicable quantum statistics, these are at least theoretically capable of achieving an internal quantum efficiency of 100%. This contrasts with diodes having purely fluorescent emitters which, on the basis of the quantum statistics of the injected charge carriers, have a maximum internal quantum yield of only 25%.
Considering the internal quantum yield alone, organic electronic components which utilize a phosphorescence-based conversion of current to light (and vice versa) (triplet emitter/emission) are thus more suitable for providing a high luminescence (cd/m2) or efficiency (cd/A, lm/W). Within the field of compounds capable of triplet emission, however, several boundary conditions have to be observed. Although phosphorescence also occurs in compounds of the elements of the fourth and fifth periods of the Periodic Table, it is the complexes of the metals of the 6th period that have become established in the abovementioned applications. According to the position of the elements in this period, the origin of the phosphorescence is weighted differently within the orbital structure of the complexes.
In the case of the lanthanoids, both the HOMO (Highest Occupied Molecular Orbital) and the LUMO (Lowest Unoccupied Molecular Orbital) are predominantly metal-centered, meaning that the proportion of the ligand orbitals is relatively minor. The effect of this is that the emission wavelength (color) of the emitters is determined almost exclusively by the band structure of the metal (examples: europium=red, terbium=green). Because of the strong shielding of the f electrons of these metals, ligands coupled to the metal are able to split the energies of the fn configuration of the metals only by about 100 cm−1, such that there is a considerable difference in the spectroscopy, by virtue of their ligand field, of the d ions from that of the f ions. In the case of ions of the lanthanides, the color results from transitions from f to unoccupied s, p and d orbitals.
Going along the period to the elements of osmium, iridium, platinum and gold, ligand fields split the metal orbitals by a factor of 10-100 times more than in the case of the lanthanoids. It is thus possible to represent virtually the entire wavelength spectrum by varying the ligands with these elements. The strong coupling of the spin angular momentum of the metal atom with the spin angular momentum of the electrons results in phosphorescence being obtained in the emitters. The HOMO is usually metal-centered, while the LUMO is usually ligand-centered. The radiative transitions are therefore referred to as metal-ligand charge transfer transitions (MLCT).
Both OLEDs and OLEECs (light-emitting organic electrochemical cells) currently utilize almost exclusively iridium complexes as emitters. In the case of the OLEDs, the emitter complexes are uncharged; in the case of the OLEECs, ionic, i.e. charged, emitter complexes are utilized. However, the use of iridium in these components has a serious disadvantage. The annual production of iridium is well below 10 t (3 t in 2000). The effect of this is that the material costs make a significant contribution to the production costs of organic electrical components. An additional factor is that iridium emitters are incapable of efficiently representing the entire spectrum of visible light. For example, stable blue iridium emitters are comparatively rare, which is a barrier to flexible use of these materials in OLEDs or OLEECs.
In the recent literature, however, there are some approaches which propose “triplet harvesting” even with non-iridium-based emitters. For example, Omary et al. in “Enhancement of the Phosphorescence of Organic Luminophores upon interaction with a Mercury Trifunctional Lewis Acid” (Mohammad A. Omary, Refaie M. Kassab, Mason R. Haneline, O. Elbjeirami, and Francois P. Gabbai, Inorg. Chem. 2003, 42, 2176-2178) point out the possibility of achieving adequate phosphorescence of purely organic emitters through the use of mercury. As a result of the heavy atom effect of mercury in a matrix composed of organic ligands, a singlet-triplet/triplet-singlet transition of the excited electrons in the organic matrix is enabled by quantum-mechanical means (ISC, intersystem crossing), which results in a distinct reduction in the lifetime of the excited electronic (triplet) states and prevents unwanted saturation of the population of these states. The cause of this mechanism is the spin-orbit coupling of the heavy mercury atom with the excited electrons of the organic matrix. A disadvantage, however, is that the use of mercury is problematic for reasons of toxicology and environmental policy.
One means of obtaining an adequate quantum yield on the basis of purely organic phosphorescence emitters is described, for example, by Bolton et al. in NATURE CHEMISTRY, 2011, 1-6 (“Activating efficient phosphorescence from purely organic materials by crystal design”, Onas Bolton, Kangwon Lee, Hyong-Jun Kim, Kevin Y. Lin and Jinsang Kim, NATURE CHEMISTRY, 2011, 1-6). This article suggests that the incorporation of heavy halides into a crystal composed of an organic matrix leads to high quantum yields through phosphorescent organic emitters. However, a disadvantage of this solution is that a particular distance between the heavy halides and the organic matrix and a crystalline structure seem to be necessary for this effect. This would be a barrier to inexpensive industrial manufacture of organic components.
WO 2012/016074 A1, by contrast, describes a thin layer comprising a compound of the formula
where Ar1 and Ar2 are each independently a C3-C30 aromatic ring; R1 and R2 are a substituent; a and b are each independently an integer from 0 to 12, where, when a is 2 or more, each R1 radical is optionally different from the others, and two R1 radicals are optionally bonded to one another to form a ring structure, and, when b is 2 or more, each R2 radical is optionally different from the others, and two R2 radicals are optionally bonded to one another to form a ring structure; A1 is any kind of direct bond, —O—, —S—, —S(═O)—, —S(═O)2—, —PR3—, —NR4— and —C(R5)2—; R3 is a hydrogen atom or a substituent; R4 is a hydrogen atom or a substituent; R5 is a hydrogen atom or a substituent and two R5 radicals are optionally different from one another; E1 is a monovalent radical having 50 or fewer carbon atoms; L1 is a ligand having 50 or fewer carbon atoms; c is an integer from 0 to 3, where, when c is 2 or more, each L1 radical is optionally different from the others; and every combination of a combination of E1 and Ar1 and a combination of E1 and Ar2 optionally forms a bond; and, when c is 1 to 3, every combination of a combination of L1 and E1, a combination of L1 and Ar1, a combination of L1 and Ar2 and a combination of L1 and L1 optionally forms a bond. A disadvantage, however, is that the compounds described have only an inadequate quantum yield and are insufficiently stable in solution, such that they decompose.
DE 103 60 681 A1 discloses main group metal diketonato complexes according to the following formula:
as phosphorescent emitter molecules in organic light-emitting diodes (OLEDs) in which M may be Tl(I), Pb(II) and Bi(III). Additionally disclosed is the use of these main group metal diketonato complexes as light-emitting layers in OLEDs, light-emitting layers comprising at least one main group metal diketonato complex, an OLED comprising this light-emitting layer, and devices comprising an OLED of the invention. In experiments, however, it was shown that the abovementioned compounds synthesized with strict exclusion of water do not exhibit phosphorescence-based emission after electronic excitation. It is highly likely that the phosphorescent emissions cited originate from indeterminate oxo clusters which have formed in an uncontrolled manner, for example as a result of hydrolysis in the course of preparation. A disadvantage of this specific solution is that the π system of these acetylacetonate ligands, especially of the fully fluorinated variants described, is not very well-developed and, as a sole phosphorescent emitter, allows only small phosphorescence yields.